1. Field of the Invention
This invention relates to an improved wireless communication system and method, and more particularly to a system and method for employing underground or low profile, surface deployed current drivers for inducing ground currents in the earth in such a way that the current drivers and earth function together as vertical plane polarized antennas which perform as loop or long-wire guided wave antennas and which send and receive vertically polarized electromagnetic signals propagated through the atmosphere over a wide bandwidth.
2. The Prior Art
Aboveground wireless communication systems have been known and used for many years. Generally, such systems employ aboveground antennas which extend high into the air for transmitting and receiving low, medium and high frequency electromagnetic signals which travel through the atmosphere. In a military sense, such aboveground communication systems are considered "soft" for security purposes because they are relatively easy to destroy. "Hardness" (or "softness") is a military term used to denote the system's vulnerability to destruction under attack. The harder a system is, the less vulnerable to destruction it is.
The hardness of a communication system is generally measured by such criteria as its ability to withstand substantial shock, as in the case of a powerful explosion occurring very near to the system and its ability to survive high energy electromagnetic pulse radiation which may be produced by a nuclear blast.
Even though a powerful explosion may be centered some distance from an aboveground communication system, the resulting shock waves will likely damage or destroy the system antennas. Furthermore, aboveground antennas which transmit or receive high frequency signals are very susceptible to the adverse effects of electromagnetic pulse radiation. Even though attempts have been made to increase the hardness of aboveground communication systems by constructing backup systems, factors such as cost and environmental considerations make it very difficult to justify and obtain the redundancy required to make such systems secure in the event of attack.
In order to increase system hardness, it is desirable to deploy communication system antennas under the ground or flat upon the ground, and in some cases it is desirable to deploy at least part of the antennas above but in close proximity to the surface of the ground. Underground and near surface above ground deployed antennas ("low profile antennas") are able to withstand the effects of nearby explosions to a much greater degree than conventional aboveground antennas. Further, such underground and surface proximity antennas are exposed to less electromagnetic pulse radiation. Because of these advantages, a communication system utilizing underground or low profile antennas requires less redundancy to achieve system security than a comparable communication system using typical aboveground antennas. However, although system hardness is improved, prior art undergroundlow profile antenna systems have been substantially less efficient in their operation than the high, aboveground antennas. Because of their poor performance characteristics, such antenna systems have had limited and very specific applications and have been wholly unable to adequately function as a replacement for the high, aboveground antenna systems.
Such inadequate performance characteristics are embodied in the various wireless subterranean signaling systems which have been proposed in the past, wherein electromagnetic signals are transmitted through the earth between underground antennas. For example, electromagnetic waves of relatively low frequencies ranging from 100 Hz to 100 KHz have been propagated through the earth between horizontally polarized electric dipole antennas buried in the earth. Such underground transmission of signals is inherently susceptible to significant signal attenuation due to the large dielectric coefficient and high conductivity of the earth. This is due to the fact that in a conductive (i.e., lossy) medium such as the earth or water, energy is dissipated through currents that are generated by the electric and magnetic field components of the wave. This energy loss results in an appreciable exponential attenuation of field strength with distance. In contrast, electromagnetic waves propagated through the atmosphere lose little energy to the medium. Thus, excess attenuation beyond inverse R.sup.2 loss is negligible in the atmospheric case except at microwave and higher frequencies.
In order to achieve system hardness while utilizing the atmosphere for signal transmission, several past proposals have involved the positioning of a dipole antenna upon or beneath the surface of the earth. Such systems have experienced significant reduction in signal strength as compared with aboveground, vertically oriented dipole antennas as a result of signal attenuation and losses in the earth.
Comparisons of the performance of subsurface dipole antennas to conventional above surface antennas are presented in Fenwick and Weeks, Submerged Antenna Characteristics, I.E.E.E. TRANSACTIONS ON ANTENNAS AND PROPAGATION, p. 296 (May, 1963), where it is seen that in many common situations the strength of the underground produced signal is more than 40 dB weaker than the signal produced by the reference antenna, which is a perfect quarter-wave vertical monopole antenna. Such reduced signal power is simply not acceptable for many communications systems applications, especially when such applications may involve long-range signal transmission. In addition to the above problem, dipole antennas produce electromagnetic signals which propagate in directions generally normal to the longitudinal axis of the dipole antenna. As a result, much of the signal strength is directed substantially straight upwards or into the ground where it is lost, resulting in significant amounts of power loss and reduced efficiency in the communication system.
In order to provide an underground antenna system while transmitting usable signals through the atmosphere, it has been proposed in the prior art to employ a buried loop antenna for generating a horizontally polarized magnetic wave which in turn generates a surface wave having a vertically polarized electric component to be received by a vertical whip receiving antenna. Although a substantial portion of the resulting signal propagates along the earth's surface, this antenna system still is very low in efficiency which is mainly a result of the use of horizontally polarized waves and the losses associated therewith. Another disadvantage of this type of prior art antenna is the very large physical antenna size needed at low and even medium range frequencies.
It becomes clear that the most efficient means for obtaining system hardness while providing for transmission and reception of electromagnetic signals through the atmosphere would be to utilize a buried wire loop or traveling wave antenna which could produce vertical plane polarized electromagnetic signals. However, it is well known that the size of such an antenna is directly related to the wavelength of the signal in earth at the frequency of operation. In fact, for optimal operation the perimeter length of the underground loop antenna should be approximately 1.4 wavelengths in earth at the operating frequency. For an operating frequency in the MF range of 400 KHz, the necessary loop antenna perimeter length would be approximately 100 meters. It becomes immediately apparent that, even if physically possible, the cost of trenching, supporting and burying such an antenna in the vertical position would make use of the antenna unrealistic if not impossible.
A further problem that is common to all underground antennas is the lower power gain which is experienced as a result of signal attenuation prior to signal entry into the atmosphere. Although this attenuation can be minimized by positioning the antenna close to the surface, it still exists in significant amounts. No adequate method has heretofore been found for substantially increasing the gain of atmospheric transmission signals emanating from an underground source and thus, this reduced performance capability has continued to be a long-standing, unresolved problem in the art.
In light of the above considerations, it is apparent that the great need that has heretofore gone unsatisfied is to provide a two-way, wireless underground or near surface aboveground deployed communication system capable of effectively receiving and transmitting signals over a wide band of frequencies, with the system being sufficiently "hard" to withstand a near miss of a nuclear weapon. The system should have reduced vulnerability to jamming and, even immediately following a nuclear explosion, should permit long distance transmission of electromagnetic signals with reasonable data rates. The system should be capable of transmitting communication signals in either broad or narrow beam configurations, and in conjunction with enhanced signal processing, should be capable of performance comparable to existing aboveground antenna systems. Furthermore, the system should feasibly permit redundancy sufficient to satisfy the need for system security without excessive costs. The underground communication systems heretofore employed have not been able to satisfy these important needs.